In the mean time I have installed a new minimal system and then I updated src and ports. What do want me to try now? Should I install the minimal system once more without updating src and ports and then rebuild the kernel using the GENERIC config file?

IIRC the last time I recompiled a FreeBSD kernel was for adding in ISDN support on FreeBSD 3.4. In those days I printed out those chapters from the FBSD handbook. You didn't want to reread those pages with a 14k4 modem with phone costs of 15 cents each 45 seconds.

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You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump

There is a difference between installing the source code and updating.

If you install the source code as part of the minimal install, you will have the exact source code that was used to build your minimal system. Then you simple can compile a custom kernel .

If you installed the source code and then updated with c(v)sup or whatever, you will have source code incorporating changes made after the release of your minimal install.
In this case you will have to read that UPDATING file and make buildworld before compiling the kernel.
I would do the GENERIC kernel first, and just follow the remainder of the steps outlined in chapter 24 of the FreeBSD handbook.
This will result in a system where the source code and the binaries are in sync again. That is the moment where you can compile your custom kernel. Capice ?

There is no secret knowledge, which only the initiated receive from the FreeBSD high priests. It is all written down in the handbook.

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You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump